Finger-scale dynamics in double-diffusive instability: Vorticity, flux, and zig-zag ascent

Salt-driven double-diffusive fingers were examined at the finger scale in a 20 cm-tall, thermally insulated, closed acrylic tank. A fluorinated-ethylene-propylene (FEP) tube at the tank base delivered inflow with a fixed temperature difference (ΔT = 5 K, cold below warm) and three salinity jumps (ΔS = 350, 450, or 550 ppm), producing an unstable stratification of cold–fresh water beneath warm–salty water. Simultaneous particle-image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) provided time-resolved velocity and concentration fields, respectively, with 35 µm PLIF resolution, 286 µm vector spacing, and 0.4 s temporal sampling. Vorticity, circulation, enstrophy, scalar-dissipation rate, and vertical and horizontal salt-flux fields were evaluated for isolated ascending fingers. During the initial growth, the fingers evolved into coherent mushroom-shaped structures whose laminar cores were bounded by thin shear layers. Peak positive circulation and enstrophy coincided with the period of maximum vertical scalar flux, thereby defining a brief interval of greatest upward buoyancy force. For the largest salinity differential (i.e., ΔS = 550 ppm), a zig-zag ascent pattern emerged: the finger drifted laterally by ~5–10 mm and relaunched vertically to generate a secondary finger at each inflection. These excursions were accompanied by bursts and a two-fold rise in horizontal salt flux relative to lower-salinity runs. The observations indicate that stronger salinity contrasts steepen the concentration gradients, intensify the vorticity production along the finger flanks, and thereby destabilize the core, which induces lateral excursions that interrupt the finger’s vertical transport.